SkyDrill

Likely source of a high frequency vibration

Aviation Mechanic Airframe (AMA) practice question 4 of 11 on Rotorcraft Fundamentals. ACS element AM.II.N.K8 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONA helicopter is reported to have a high frequency vibration. Which component is a likely source?

  1. AThe main rotor mast and transmission
  2. BA loose skid on the airframe
  3. CThe tail rotor drive shaft
Show the answer and explanation

Correct answer: C. The tail rotor drive shaft

  • A — incorrect The rotor, mast, and transmission system produce pylon rock at two to three cycles per second, which falls in the extreme low frequency range.
  • B — incorrect A loose airframe component such as a skid vibrating at 4, 5, or 6 per revolution produces medium frequency vibration, not high frequency.
  • C — correct High frequency vibration typically occurs when the tail rotor gears, tail drive shaft, or tail rotor fan or shaft assembly vibrates or rotates at a speed equal to or greater than the tail rotor.

BACKGROUND

Helicopter vibrations are grouped by frequency level. Extreme low frequency vibration is essentially limited to pylon rock at two to three cycles per second, inherent in the rotor, mast, and transmission system, and transmission mount dampening is fitted to absorb it. Low frequency vibrations of 1 per revolution and 2 per revolution come from the rotor itself; a 1 per revolution vibration is caused simply by one blade developing more lift at a given point than another blade develops at the same point. Medium frequency vibration, generally 4, 5, or 6 per revolution, is inherent in most rotors and grows when the fuselage loses some ability to absorb vibration or when an airframe component such as a skid works loose. High frequency vibration is caused by anything that rotates or vibrates at extremely high speed, typically the tail rotor gears, tail drive shaft, or the tail rotor fan or shaft assembly turning at or above tail rotor speed.

STUDY TIP

Turn the frequency into a place to look: below 1 per revolution points at the pylon, 1 or 2 per revolution at the main rotor, 4 to 6 per revolution at the airframe, and anything faster at the tail drive system.

KEY POINTS

  • High frequency vibration comes from parts turning at or above tail rotor speed, such as the tail drive shaft and tail rotor gears.
  • Pylon rock from the rotor, mast, and transmission is extreme low frequency at two to three cycles per second.
  • Medium frequency vibration of 4, 5, or 6 per revolution points to the airframe, including a loose skid.

SOURCES

  • FAA-H-8083-31B ch02 "Helicopter Vibration — High Frequency Vibration" (sources/ama-airframe/ch02.txt)
Practice the Aviation Mechanic Airframe (AMA) test →Timed 100-question exam, by ACS area, or review your misses. Free.
MORE ON ROTORCRAFT FUNDAMENTALS

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-31B Aviation Maintenance Technician Handbook – Airframe, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.